Multi-block polymer comprising a urea prepolymer chain extended with a compatible second prepolymer, the membrane made therefrom and its use in separations

ABSTRACT

The present invention is directed to a multi-block polymeric material comprising a urea prepolymer chain extended with a second compatible prepolymer selected from the group of prepolymers comprising (a) an (A) dianhydride or its corresponding tetraacid or diacid-diester combined with a monomer selected from (B) epoxy, diisocyanate, polyester, and diamine in an A/B mole ratio ranging from about 2.0 to 1.05, preferably about 2.0 to 1.1, and (b) an (A) diamine combined with a monomer selected from (B) epoxy and dianhydride or its corresponding tetraacid or diacid-diester in an A/B mole ratio ranging from about 2.0 to 1.05, preferably about 2.0 to 1.1, and mixtures thereof. The present invention is also directed to membranes of the above recited multi-block polymeric material, especially membranes comprising them, dense films of said multi-block polymeric material deposited on a microporous support layer producing a thin film composite membrane. The membranes of the multi block polymeric material, especially the thin film composite membranes are useful for separating aromatic hydrocarbons from mixtures of same with non-aromatic hydrocarbons under perstraction or pervaporation conditions.

This is a division, of application Ser. No. 624,160, filed Dec. 6, 1990,now U.S. Pat. No. 5,039,422.

BACKGROUND OF THE INVENTION

Polyurea/urethane membranes and their use for the separation ofaromatics from non-aromatics are the subject of U.S. Pat. No. 4,914,064.In that case the polyurea/urethane membrane is made from apolyurea/urethane polymer characterized by possessing a urea index of atleast about 20% but less than 100%, an aromatic carbon content of atleast about 15 mole percent, a functional group density of at leastabout 10 per 1000 grams of polymer, and a C═O/NH ratio of less thanabout 8.0. The polyurea/urethane multi-block copolymer is produced byreacting dihydroxy or polyhydroxy compounds, such as polyethers orpolyesters having molecular weights in the range of about 500 to 5,000with aliphatic, alkylaromatic or aromatic diisocyanates to produce aprepolymer which is then chain extended using diamines, polyamines oramino alcohols. The membranes are used to separate aromatics fromnon-aromatics under perstraction or pervaporation conditions.

The use of polyurethane-imide membranes for aromatics from non-aromaticsseparations is disclosed in U.S. Pat. No. 4,929,358. Thepolyurethane-imide membrane is made from a polyurethane-imide copolymerproduced by end capping a polyol such as a dihydroxy or polyhydroxycompound (e.g. polyether or polyester) with a di or polyisocyanate toproduce a prepolymer which is then chain extended by reaction of saidprepolymer with a di or polyanhydride or with a di or polycarboxylicacid to produce a polyurethane amic acid which is then chemically orthermally cyclized to the imide. The aromatic/non-aromatic separationusing said membrane is preferably conducted under perstraction orpervaporation conditions.

A polyester imide copolymer membrane and its use for the separation ofaromatics from non-aromatics is the subject of U.S. Pat. No. 4,946,594.In that case the polyester imide is prepared by reacting polyester witha dianhydride to produce a prepolymer which is then chain extended witha diisocyanate to produce the polyester imide.

The use of membranes to separate aromatics from saturates has long beenpursued by the scientific and industrial community and is the subject ofnumerous patents.

U.S. Pat. No. 3,370,102 describes a general process for separating afeed into a permeate stream and a retentate stream and utilizes a sweepliquid to remove the permeate from the face of the membrane to therebymaintain the concentration gradient driving force. The process can beused to separate a wide variety of mixtures including various petroleumfractions, naphthas, oils, hydrocarbon mixtures. Expressly recited isthe separation of aromatics from kerosene.

U.S. Pat. No. 2,958,656 teaches the separation of hydrocarbons by type,i.e. aromatic, unsaturated, saturated, by permeating a portion of themixture through a non-porous cellulose ether membrane and removingpermeate from the permeate side of the membrane using a sweep gas orliquid. Feeds include hydrocarbon mixtures, naphtha (including virginnaphtha, naphtha from thermal or catalytic cracking, etc.).

U.S. Pat. No. 2,930,754 teaches a method for separating hydrocarbonse.g. aromatic and/or olefins from gasoline boiling range mixtures, bythe selective permeation of the aromatic through certain cellulose esternon-porous membranes. The permeated hydrocarbons are continuouslyremoved from the permeate zone using a sweep gas or liquid.

U.S. Pat. No. 4,115,465 teaches the use of polyurethane membranes toselectively separate aromatics from saturates via pervaporation.

DETAILED DESCRIPTION OF THE INVENTION

The present invention comprises a multi-block polymeric materialcomprising a first prepolymer made by combining (A) a diisocyanate with(B) a diamine in an A/B or B/A mole ratio ranging from about 2.0 to1.05, preferably about 2.0 to 1.1 to produce a urea prepolymer which issubsequently chain extended in an about 1 to 1 mole ratio with a second,different, and compatible prepolymer selected from the group ofprepolymers comprising (a) an (A) dianhydride or its correspondingtetraacid or diacid-diester combined with a monomer selected from (B)epoxy, diisocyanate, polyester, and diamine in an A/B mole ratio rangingfrom about 2.0 to 1.05, preferably about 2.0 to 1.1, and (b) an (A)diamine combined with a monomer selected from (B) epoxy and dianhydrideor its corresponding tetra acid or di acid-diester in an A/B mole ratioranging from about 2.0 to 1.05, preferably about 2.0 to 1.1, andmixtures thereof.

When the first prepolymer urea is produced using diisocyanate anddiamine in an about 1.05 to 2 mole ratio, the compatible secondprepolymer with which it is reacted cannot itself be end-capped with adiamine. Thus, for such 1.05/2 mole ratio diisocyanate-diamine firstprepolymer, second prepolymers made from diamine to epoxy or dianhydrideor corresponding tetra acid or di acid-diester mole ratios of greaterthan 1 are not compatible second prepolymers. For the purposes of thisspecification and the following claims, the above definition andexclusion are what are to be understood when the term compatible secondprepolymer is used.

The multi-block prepolymer material can be formed into a thick membranelayer or deposited as a thin, dense (nonporous) film on a micro-poroussupport resulting in the production of a thin film composite membrane.

The membranes can be used to separate aromatic hydrocarbons includingheteroatom containing aromatics from mixtures of same with non-aromatichydrocarbons under perstraction or pervaporation conditions. As usedhereinafter in this text and the appended claims the term "aromatichydrocarbons" is understood as meaning to include single and multi-ringside chain bearing and unsubstituted aromatics containing only carbonand hydrogen, single and multi-ring side chain bearing and unsubstitutedheterocyclic aromatics such as thiophene, pyridine, quinoline,benzothiophene, benzofuran, etc., and single and multi-ring aromatic andheterocyclic aromatics bearing heteroatom substituted side chains.

In preparing the multi-block polymeric material one begins by preparinga first prepolymer made by combining (A) a diisocyanate with (B) adiamine in an A/B or B/A mole ratio ranging from about 2.0 to 1.05,preferably about 2.0 to 1.1. The prepolymer is identified as being aurea.

Aliphatic and cycloaliphatic di and poly isocyanate can be used as canbe mixtures of aliphatic, cycloaliphatic, aralkyl and aromaticpolyisocyanates.

The diisocyanates are preferably aromatic diisocyanates having thegeneral structure: ##STR1## wherein R' and R" are the same or differentand are selected from the group consisting of H, C₁ -C₅ and C₆ H₅ andmixtures thereof and n ranges from 0 to 4.

Aliphatic diisocyanates which may be utilized are exemplified byhexamethylene diisocyanate (HDI),1,6-diisocyanato-2,2,4,4-tetramethylhexane (TMDI), 1,4-cyclohexanyldiisocyanate (CHDI), isophorone diisocyanate (IPDI), while usefulalkylaromatic diisocyanates are exemplified by 2,4-toluene diisocyanate(TDI) and bitolylene diisocyanate (TODI). Aromatic diisocyanates areexemplified by 4,4'-diisocyanate diphenylmethane (MDI), methylenedichlorophenyl diisocyanate (dichloro MDI), methylene dicyclohexyldiisocyanate (H₁₂ MDI), methylene bis [dichlorophenylisocyanate](tetrachloro MDI), and methylene bis [dichlorocyclohexylisocyanate] (tetrachloro H₁₂ MDI). polyisocyanates are exemplified bypolymeric MDI (PMDI) and carbodiimide modified MDI and isocyanurateisocyanates.

The di or polyisocyanate is combined with a diamine.

Diamines which can be used have the general formula H₂ NRNH₂ where Rincludes aliphatic and aromatic moieties, such as ##STR2## where n is 1to 10 and R' may be the same or different and are selected from thegroup consisting of H, C₁ -C₅ carbons and C₆ H₅ and mixtures thereof.

Also included are diamines of the formula: ##STR3## where R', R'' andR''' are the same or different and are selected from the groupconsisting of H or Cl or a C₁ to C₅ or C₆ H₅ and mixtures thereof and nranges from 0 to 4.

Useful polyamines are exemplified by polyethyleneimines and 2,2',2''triaminotriethylamine.

The diisocyanate and diamine are combined in an about 2/1 to 1/2 moleratio. The reaction between these reactants is a room temperaturecondensation reaction which is permitted to run to completion.Completion can be determined by any technique well known to thoseskilled in the art. For example ASTM technique D2572 can be used.Similarly the disappearance of isocyanate groups can be monitored byinfrared spectroscopy.

The second compatible prepolymer which is added to the urea is selectedfrom the group of prepolymers comprising (a) an (A) dianhydride or itscorresponding tetraacid or diacid-diester combined with a monomerselected from (B) epoxy, diisocyanate, polyester and diamine in an A/Bmole ratio ranging from about 2 to 1.05, preferably about 2.0 to 1.1,and (b) an (A) diamine combined with a monomer selected from (B) epoxyand dianhydride or its corresponding tetraacid or diacid-diester in anA/B mole ratio ranging from about 2 to 1.05, preferably about 2.0 to1.1, and mixtures thereof.

The diisocyanates used in preparing this second compatible prepolymer isselected from the same group of diisocyanates described previously forthe production of the urea.

The diamines used in preparing this second compatible prepolymer arealso selected from the same group as previously described for theproduction of the urea.

The epoxy used has the general formula: ##STR4## R may be any saturated,unsaturated, or aromatic group, halogen substituted saturated,unsaturated or aromatic group as well as groups containing oxygen in theform of ether linkages, and mixtures thereof.

Representative of useful epoxy compounds are the following: ##STR5##identified as DER332 from Dow Chemical ##STR6## identified as DER542from Dow Chemical

Polyesters having molecular weights in the range of about 500 to 5000can be used in preparing the second compatible prepolymer.

The polyester components are prepared from aliphatic or aromaticdicarboxylic acids and aliphatic or aromatic dialcohols. Aliphaticdicarboxylic acids refer to those materials having the general formulaHOOCRCOOH where R contains 2 to 10 carbons (and may be either a straightor branched chain configuration). Aromatic dicarboxylic acids refer tothose materials having the general structure HOOCRCOOH where R is:##STR7## wherein R', R'', and R''' may be the same or different and areselected from the group consisting of H and C₁ -C₅ carbons or C₆ H₅ andcombinations thereof, and n is 0 to 4. It is to be understood that inthe above formula each R' or R'' may itself represent a mixture of H, C₁-C₅ or C₆ H₅.

Dialcohols have the general structure HOROH where R may be ##STR8##where n is 1 to 10, preferably 4 to 6, and R' is H, C₁ to C₅ or C₆ H₅ or##STR9## where R', R'', R''' and n are defined in the same manner as forthe aromatic dicarboxylic acids. An example of a useful dialcohol isbisphenol A.

Dianhydrides or tetra carboxylic acids or di acid-diesters which produceamide acid groups are also used in producing the second prepolymer.

Any aromatic, aliphatic, cycloaliphatic or araliphatic dianhydride canbe used. Examples of di anhydrides include by way of example and notlimitation: pyromellitic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride,4,4'-(hexafluoroisopropylidene)-bis-(phthalic anhydride),4,4'-oxydiphthalic anhydride, diphenylsulfone-3,3'4,4'-tetracarboxylicdianhydride, and 3,3',4,4'-biphenyltetracarboxylic dianhydride.

When dicarboxylic acid/diester and tetracarboxylic acid derivatives ofdianhydrides are used they must first be converted to species that willreact with diamines or polyesters. This can be done by conversion of thedicarboxylic acid/diester or tetracarboxylic moieties to (1) acidchlorides via derivatization with e.g. thionyl chloride or to (2)diimidazoles via reaction with e.g. carbonyl diimidazole. Subsequentreaction of the derivatized prepolymer with (1) diamines results information of an amide acid which must then be thermally or chemicallycyclized to form the imide, or (2) polyesters results in formation of anester which requires no further curing.

In each instance the appropriate monomeric materials in theaforementioned mole ratios are combined to produce the desired secondprepolymer. Solvents are employed if needed The selection of a propersolvent is left to the practitioner

Depending on the physical nature of the second prepolymer the reagentsare combined and either reacted to completion or to a point short ofcompletion. The reaction is run to completion when such secondprepolymer exists in liquid or solution in solvent form. If however thesecond prepolymer when run to completion is in the form of a solid orinsoluble gel then reaction to completion is unacceptable. In suchinstances the reagents are reacted until just before the viscosity ofthe reaction mixture becomes too difficult to manage. The secondprepolymer is then, combined with the first prepolymer.

The first and second prepolymers can be reacted neat, that is, in theabsence of added solvent, if their individual natures favor such absenceof solvent, or the reaction can be run in the presence of a solventappropriate for the polymerization conditions employed. In general thereaction will be run in a solvent which may be selected from any of thepolar, aprotic solvents such as tetrahydrofuran (THF), DMAC, DMSO, DMF,as well as NMP and cellosolve acetate.

Certain combinations of first prepolymer and second prepolymer may bereacted to completion while other combinations must be used to cast amembrane before the reaction goes to completion, i.e., while thereactant solution is still of a manageable viscosity and beforeformation of a gel. In those instances, which can be determined by thepractitioner using the information before him in this specificationwithout the expenditure of any inventive effort, the solution is spreador poured on the appropriate support, the copolymer layer on supportinserted into an oven to drive off the casting solution solvent, thenheated to a temperature for a time sufficient to drive thepolymerization reaction to completion and cure the membrane.

The multi-block polymer in solvent or in added solvent if needed is usedas a casting solution. Polymer concentration in solvent ranges from 10to 70 wt % preferably 15-50 wt % for casting dense films. When castingintegral thin film composite membranes, e.g. thin, dense (nonporous)layers of multi-block polymer preferably about 0.1 to 5.0 micron inthickness on porous support backings such as ceramic, sintered glass ormetal or polymeric material such as ceramics or sintered glass or metalor polymeric supports such as nylon, porous polypropylene, porousTeflon®, or porous urea, preferably porous Teflon® the polymerconcentration in solution is on the order of about 50% or less.

The casting solution is poured or spread on an appropriate supportmedium, such as a metal or glass plate or, if desired, a woven fiberbacking, such as woven fiber glass, nylon, polyester, etc. can be usedif solvent removal during the casting sequence employs a vacuum, butpreferably, non-woven backings such as thin films of porouspolypropylene, porous urea or porous Teflon® are employed. In general,however, backing materials used are those which are not attacked by thesolvent(s) used to produce the copolymer casting solution and which cansurvive in the environment (chemical and thermal) to which the membranewill be exposed.

The membrane may be cast in any thickness, membranes ranging inthickness of from about 0.1 to about 50 microns being preferred, thethin, dense layer in the composite membrane being preferably about 0.1to 5.0 microns thick.

A very thin layer of the multi-block polymer can be deposited onto ahighly permeable, non-selective layer producing a composite membranecomprising a thin, dense, (nonporous) layer of multi-block copolymermembrane preferably about 0 1 to 5 microns thick on a permeable,non-selective, thick backing. The thick underlayer (about 20 to 100microns thick) serves as a support layer permitting one to produce thin,dense, selective layers of multi-block polymer membranes which wouldotherwise be mechanically unmanageable due to their thinness. In manyinstances due to the chemical similarity between the support layer andurea containing selective layer, the two layers interact throughhydrogen bonding to produce a very strong bond in addition to physicaladhesion For general, low temperature applications the porous,non-selective backing need not itself be capable of operation at hightemperatures. In such service, such as the perstractive separation ofaromatics from non-aromatics, backings such as polyurethane orpolyurea/urethane would be sufficient. For higher temperatureapplications, of course, the backing material must itself be capable ofremaining intact at the high temperature. For such applications backingssuch as polyester/imides, teflon or even ceramics, sintered glass ormetal supports should be used.

If one were to use this technique to produce sheet material, the thick,permeable underlayer such as polyurethane can be deposited on a suitablecasting backing material such as glass, metal, porous fiber glass,polyethylene, polypropylene, nylon, Teflon®, etc. after which the thin,dense selective layer would be deposited onto the under layer. Thecasting backing material can then be removed leaving the composite sheetmembrane.

In producing hollow fibers or tubes using this composite membranetechnique, first a tube or fiber of permeable material such aspolyurethane is produced after which a thin dense layer of themultiblock polymer material is deposited on either the outer or innersurface of the tube or fiber support.

The permeable polyurethane layer can be prepared from polyether glycolssuch as polypropylene glycol or polybutylene glycol plus aliphaticand/or aromatic diisocyanates (preferably aliphatic diisocyanates) usingpolyols (diols or triols) preferably aliphatic diols as chain extenders.Polyurethane membrane materials which satisfy the above requirement ofpermeability are the polyurethane membranes described in U.S. Pat. No.4,115,465.

The membranes are useful for the separation of aromatics fromnon-aromatics in petroleum and chemical streams, and have been found tobe particularly useful for the separation of larger, substitutedaromatics from non-aromatics as are encountered in heavy cat naphthastreams. Other streams which are also suitable feed streams foraromatics from saturates separation are intermediate cat naphtha streams(200°-320° F.), light aromatics content streams boiling in the C₅ -300°F. range, light catalytic cycle oil boiling in the 400°-650° F. range,reformate streams as well as streams in chemical plants which containrecoverable quantities of benzene, toluene, xylene (BTX) or otheraromatics in combination with saturates. The separation techniques whichmay successfully employ the membranes of the present invention includeperstraction and pervaporation.

Perstraction involves the selective dissolution of particular componentscontained in a mixture into the membrane, the diffusion of thosecomponents through the membrane and the removal of the diffusedcomponents from the downstream side of the membrane by use of a liquidsweep stream. In the perstractive separation of aromatics from saturatesin petroleum or chemical streams (particularly heavy cat naphthastreams) the aromatic molecules present in the feedstream dissolve intothe membrane film due to similarities between the membrane solubilityparameter and those of the aromatic species in the feed. The aromaticsthen permeate (diffuse) through the membrane and are swept away by asweep liquid which is low in aromatics content. This keeps theconcentration of aromatics at the permeate side of the membrane film lowand maintains the concentration gradient which is responsible for thepermeation of the aromatics through the membrane.

The sweep liquid is low in aromatics content so as not to itselfdecrease the concentration gradient. The sweep liquid is preferably asaturated hydrocarbon liquid with a boiling point much lower or muchhigher than that of the permeated aromatics. This is to facilitateseparation, as by simple distillation. Suitable sweep liquids,therefore, would include, for example, C₃ to C₆ saturated hydrocarbonsand lube basestocks (C₁₅ -C₂₀).

The perstraction process is run at any convenient temperature,preferably as low as possible.

The choice of pressure is not critical since the perstraction process isnot dependent on pressure, but on the ability of the aromatic componentsin the feed to dissolve into and migrate through the membrane under aconcentration driving force. Consequently, any convenient pressure maybe employed, the lower the better to avoid undesirable compaction, ifthe membrane is supported on a porous backing, or rupture of themembrane, if it is not.

If C₃ or C₄ sweep liquids are used at 25° C. or above in liquid state,the pressure must be increased to keep them in the liquid phase.

Pervaporation, by comparison, is run at generally higher temperaturesthan perstraction with the feed being in either liquid or vapor form andrelies on vacuum or sweep was on the permeate side to evaporate orotherwise remove the permeate from the surface of the membrane andmaintain the concentration gradient driving force which drives theseparation process. As in perstraction, the aromatic molecules presentin the feed dissolve into the membrane film, migrate through said filmand reemerge on the permeate side under the influence of a concentrationgradient. Pervaporation separation of aromatics from saturates can beperformed at a temperature of about 25° C. for the separation of benzenefrom hexane but for separation of heavier aromatic/saturate mixtures,such as heavy cat naphtha, higher temperatures of at least 80° C. andhigher, preferably at least 100° C. and higher, more preferably 120° C.and higher (up to about 170° to 200° C. and higher) can be used, themaximum upper limit being that temperature at which the membrane isphysically damaged. Vacuum on the order of 1-50 mm Hg is pulled on thepermeate side. The vacuum stream containing the permeate is cooled tocondense out the highly aromatic permeate. Condensation temperatureshould be below the dew point of the permeate at a given vacuum level.

The membrane itself may be in any convenient form utilizing anyconvenient module design. Thus, sheets of membrane material may be usedin spiral wound or plate and frame permeation cell modules. Tubes andhollow fibers of membranes may be used in bundled configurations witheither the feed or the sweep liquid (or vacuum) in the internal space ofthe tube or fiber, the other material obviously being on the other side.

Most conveniently, the membrane is used in a hollow fiber configurationwith the feed introduced on the exterior side of the fiber, the sweepliquid flowing on the inside of the hollow fiber to sweep away thepermeated highly aromatic species, thereby maintaining the desiredconcentration gradient. The sweep liquid, along with aromatics containedtherein, is passed to separation means, typically distillation means,however, if a sweep liquid of low enough molecular weight is used, suchas liquefied propane or butane, the sweep liquid can be permitted tosimply evaporate, the liquid aromatics being recovered and the gaseouspropane or butane (for example) being recovered and reliquefied byapplication of pressure or lowering the temperature.

The present invention is demonstrated by the following non-limitingexamples.

EXAMPLE 1

A urea prepolymer was formed by weighing 2.63 grams (0.01 mole) DesmodurW (H₁₂ MDI) and 1.24 grams (0.005 mole) 3,3'-diaminodiphenyl sulfoneinto a round bottom flask equipped with a stirrer and blanketed undernitrogen. To this was added 3.87 grams dimethylformamide and thesolution was stirred at 100° C. for 2 hours.

EXAMPLE 2

An ester prepolymer was prepared by adding 25.28 grams (0.05 mole)polyethylene adipate (500 MW) and 21.81 grams (0.1 mole) pyromelliticdianhydride to another round bottom flask similarly equipped. To thiswas added 20 grams dimethylformamide and the solution was heated andstirred at 100° C. for 4 hours.

EXAMPLE 3

Six point seventy-one (6.71) grams (0.005 mole) of the ester fromExample 2 were then transferred to the flask containing all of the ureaprepolymer from Example 1 and the resulting mixture was heated at 95° C.for 2 hours.

A membrane was cast from the resulting polymer onto a porous Teflon®backing using a 10 mil casting knife. The film was air dried at roomtemperature over the weekend, then dried at 100° C. under nitrogen purgeand finally dried at 100° C. under hard vacuum. Air permeability, whichis used as a measure of pinhole defects, was 0 at both 3 psig and 30psig. A piece of the film was subsequently evaluated foraromatic/saturate separation. Micrometer measurements showed that it wasapproximately 54 microns thick.

EXAMPLE 4

A second urea prepolymer was formed by weighing 12.5 grams (0.05 mole)methylene diisocyanate and 6.2 grams (0.025 mole) 4,4'-diaminodiphenylsulfone into a round bottom flask equipped with a stirrer and blanketedunder nitrogen. To this was added 74.8 grams dimethylformamide and thesolution was stirred at 25° C. for 40 minutes.

EXAMPLE 5

Another ester prepolymer was prepared by adding 99.5 grams (0.05 mole)polyethylene adipate (2000 MW) and 21.81 grams (0.1 mole) pyromelliticdianhydride to another round bottom flask similarly equipped to that inExample 1. To this was added 121.3 grams dimethylformamide and thesolution was heated and stirred at 100° C. for 2 hours.

EXAMPLE 6

Eighteen point seven (18.7) grams of urea prepolymer from Example 4(0.01 mole) were then placed in a syringe and added slowly to a flaskcontaining 24.2 grams (0.01 mole) of the ester prepolymer from Example 5and the resulting mixture was stirred at 25° C. for 45 minutes. It wasthen heated to 100° C. for 3 hours with stirring.

A membrane was cast from the resulting polymer onto a porous Teflon®backing using a 10 mil casting knife. The film was air dried overnight,then dried at 100° C. under nitrogen purge overnight and finally driedat 100° C. under hard vacuum for 15 minutes. Air permeability, which isused as a measure of pinhole defects, was 0 at both 3 psig and 30 psig.A piece of the film was subsequently evaluated for aromatic/saturateseparation. Micrometer measurements showed that it was approximately 44microns thick. The membrane was subsequently evaluated foraromatic/saturate separation.

EXAMPLE 7

Pervaporation experiments were carried out using membranes from Examples3 and 6. The feed for these evaluations consisted of 10 wt % toluene, 40wt % p-xylene, 30 wt % n-octane and 20 wt % isooctane. It was pumped ata rate of approximately 1 cc/minute through a preheat coil into a heatedpervaporation cell containing approximately 5.1 cm² of effectivemembrane area. Temperature was accurately maintained within 1° C. of thesetpoint. Vacuum on the permeate side was maintained with a small vacuumpump at approximately 20 torr and permeate samples were condensed intosample receivers immersed in dry ice-acetone cold traps. Results fromthe runs made are shown in Table 1. As can be seen, the selectivity foraromatics over saturates is very good.

Selectivity in these experiments was calculated by the followingformula: ##EQU1##

                  TABLE 1                                                         ______________________________________                                        Membrane       Example 3   Example 6                                          ______________________________________                                        Temperature (°C.)                                                                     150     175     100   150                                      Vacuum (torr)  20      20      20    20                                       Permeability (kg-μ/m.sup.2 /d)                                                            54      520     665   2087                                     Selectivity                                                                   Toluene/isooctane                                                                            32.7    18.2    12.12 7.57                                     Total aromatics/saturates                                                                    9.44    7.91    6.55  4.88                                     ______________________________________                                    

EXAMPLE 8

A urea prepolymer was prepared as follows. To 0.70 gram (0.004 mole)toluene diisocyanate was added 1.21 grams N-methyl pyrrolidone (NMP).Separately, 0.36 gram (0.002 mole) diethyltoluene diamine was added to3.0 grams NMP. The two solutions were added together and allowed toreact at room temperature to form the urea prepolymer after which 0.04gram LiCl was added to keep the prepolymer in solution.

EXAMPLE 9

Another prepolymer constituting 2 moles of a diamine reacted with onemole of epoxy was prepared by adding 20.92 grams (102 meq NH₂) of2,2'-Bis[4-[4-aminophenoxy)phenyl)propane (BAPP) to 29.56 gra To thissolution was added 8.69 grams (50 meq epoxy) diglycidyl ether ofBisphenol-A (DER-332) and the mixture was stirred and heated for 5 hoursat 51° C.

EXAMPLE 10

To 2.79 grams of the prepolymer from Example 9 were added 4.18 grams NMPto reduce the concentration to 20 wt. %. This latter portion was addedto the entire amount of the prepolymer from Example 8. The mixture wasspread into a thin layer onto microporous Teflon® and dried undernitrogen at room temperature overnight. It was then heated to 100° C.under nitrogen flow for 1 hour and post-cured at 180° C. for 4 hours. Apiece of the film, cut out for testing, measured 166 microns. Testingunder pervaporation conditions using a model feed consisting of 10 wt. %toluene, 40 wt. % p-xylene, 20 wt. % isooctane and 30 wt. % n-octaneproduced the results shown below in Table II.

                  TABLE II                                                        ______________________________________                                        Tempera-                                                                             Selectivity            Permeability                                    ture (°C.)                                                                    Toluene/n-Octane                                                                           p-Xylene/n-Octane                                                                           (kg-μ/m.sup.2 /d)                        ______________________________________                                        140    14.2         8.45           7.6                                        150    12.7         7.89          13.0                                        160    10.3         8.71          21.4                                        180    10.0         7.32          88.8                                        ______________________________________                                    

EXAMPLE 11

Another urea prepolymer was made as follows: to 1.74 grams (0.01 mole)TDI was added 1.74 grams NMP. To 4.25 grams (0.005 mole) of a ureacondensate, made by Texaco and known as Jeffamine DU-700, were added4.25 grams NMP. The two solutions were added together, mixed well andallowed to react for 1 hour at room temperature.

EXAMPLE 12

Another prepolymer consisting of two moles of a dianhydride and one moleof a diisocyanate was prepared as follows: 6.20 grams (0.02 mole)oxydiphthalic anhydride (ODPA) were dissolved into 30.02 grams NMP.Separately, 1.74 grams (0.01 mole) TDI was mixed with 1.74 grams NMP.The two solutions were added together and heated with stirring at 100°C. for 2.5 hours until evolution of CO₂ had ceased.

EXAMPLE 13

Approximately 3.01 grams of the prepolymer from Example 11 were added to5.0 grams of the prepolymer from Example 12. The mixture was stirred andheated to 100° C. for 1.75 hours until no CO₂ evolution was evident. Thesolution was poured into a Teflon® mold and the solvent evaporated at148° C. under flowing nitrogen overnight. A piece of the film was cutout for pervaporation testing and measured 172 microns in thickness.Pervaporation testing was carried out using the model feed described inExample 6 and the results are shown in Table III.

                  TABLE III                                                       ______________________________________                                        Tempera-                                                                             Selectivity            Permeability                                    ture (°C.)                                                                    Toluene/n-Octane                                                                           p-Xylene/n-Octane                                                                           (kg-μ/m.sup.2 /d)                        ______________________________________                                        80     5.40         3.63           62                                         90     5.24         3.64          101                                         100    4.88         3.46          168                                         ______________________________________                                    

EXAMPLE 14

Another urea prepolymer was made this time using a diisocyanate and atriamine so that the final membrane structure would be crosslinked.Approximately 1.68 grams (12.8 meq. NCO) 4,4'diisocyanato dicyclohexylmethane were mixed with 12.06 grams NMP. Subsequently, 1.01 grams (6.4meq. NH₂) Jeffamine T-403, a triamine, was added to 12.06 grams NMP. Thetwo solutions were added together quickly with stirring and used withinone hour of mixing.

EXAMPLE 15

Approximately 4.68 grams of urea prepolymer from Example 14 were mixedwith 2.00 grams of prepolymer from Example 12 and the mixture was pouredinto a Teflon® mold. The mold was placed into an oven at 148° C. underflowing nitrogen for 22 hours, then removed and cooled.

EXAMPLE 16

The membrane from Example 15 was mounted into a pervaporation cell andtested with the model feed from Example 7. Thickness of the membrane wasapproximately 107 microns. Results of the pervaporation test are shownbelow in Table IV.

                  TABLE IV                                                        ______________________________________                                        Tempera-                                                                             Selectivity            Permeability                                    ture (°C.)                                                                    Toluene/n-Octane                                                                           p-Xylene/n-Octane                                                                           (kg-μ/m.sup.2 /d)                        ______________________________________                                        130    9.64         6.06           38                                         140    8.99         5.77           67                                         150    8.36         5.56          113                                         160    7.61         5.19          181                                         ______________________________________                                    

As can be seen from these examples, all of these membranes are effectivefor the separation of aromatics from non-aromatics.

What is claimed is:
 1. A membrane made of a multi-block polymercomprising a first prepolymer urea made by combining a diisocyanate with(B) a diamine in an A/B or B/A mole ratio ranging from about 2.0 to1.05, chain extended in an about 1 to 1 mole ratio with a second,different and compatible prepolymer selected from the group ofprepolymers comprising (a) an (A) dianhydride or its correspondingtetraacid or diacid-diester combined with a monomer selected from (B)epoxy, diisocyanate, polyester, and diamine in an A/B mole ratio rangingfrom about 2.0 to 1.05, and (b) an (A) diamine combined with a monomerselected from (B) epoxy and dianhydride or its corresponding tetra-acidor di acid-diester in an A/B mole ratio ranging from about 2.0 to 1.05,and mixtures thereof.
 2. The membrane of claim 1 comprising a thin,dense layer of said multi-block polymeric material deposited on amicroporous support layer producing a thin film composite membrane. 3.The composite membrane of claim 2 wherein the microporous support layeris nylon, porous polypropylene, porous Teflon®, porous polyurea orporous polyurethane.
 4. The membrane of claim 1 wherein the membranelayer ranges from about 0.1 to about 50 microns in thickness.
 5. Thecomposite membrane of claim 2 or 3 wherein the thin dense layer ofmulti-block polymeric material ranges from about 0.1 to about 5.0microns in thickness.
 6. A multi-block polymer comprising a firstprepolymer urea made by combining a diisocyanate with (B) a diamine inan A/B or B/A mole ratio ranging from about 2.0 to 1.05, chain extendedin an about 1 to 1 mole ratio with a second, different and compatibleprepolymer selected from the group of prepolymers comprising (a) an (A)dianhydride or its corresponding tetraacid or diacid-diester combinedwith a monomer selected from (B) epoxy, diisocyanate, polyester, anddiamine in an A/B mole ratio ranging from about 2.0 to 1.05, and (b) an(A) diamine combined with a monomer selected from (B) epoxy anddianhydride or its corresponding tetraacid or diacid-diester in an A/Bmole ratio ranging from about 2.0 to 1.05, and mixtures thereof.